The best actors are the one who "believe they are the person they are portraying." This type of acting is also known as "method acting."Daniel Day-Lewis is considered one of the best actors when "living the character".He is famous for continue "acting" for the whole length of the shoot (which on a Hollywood movie can last from 2 to 6 months) -when he portrayed Abraham Lincoln on the "Lincoln", he "acted" as Abraham for months - When he would go back to his trailer/hotel to rest, he continued "on-character".Here is a great interview with him:

Who is your favorite actor and why?